Course and distribution of efferent fibers in the cochlea of the mouse.

The course, distribution and termination of single efferent fibers to the cochlea has been described in only a few animals and relatively few fibers have been studied with knowledge of their ipsilateral or contralateral origin. In order to examine the efferent fibers in the mouse, the anterograde tracer Phaseolus vulgaris leucoagglutinin (PHA-L) was iontophoretically injected into one side of the brain stem near the location of known efferent nuclei. Examination of surface preparations of the cochlea revealed detailed information for both the lateral olivocochlear (LOC) and medial olivocochlear (MOC) systems. Many, but not all, fibers entered the cochlea within the intraganglionic spiral bundle (IGSB). The LOC fibers were restricted to the ipsilateral cochlea and rarely branched within the IGSB and osseous spiral lamina (OSL). In the organ of Corti, they traveled either basally or apically in the region of the inner hair cells (IHCs), spanning lengths up to 130 microns (basally) and 890 microns (apically). Terminal swellings of these fibers were ca 3.0 microns in diameter. Numerous en passant swellings were present where the fibers formed a plexus in the area of the IHCs. The MOC fibers followed a similar course in the IGSB and OSL, and within the OSL the fibers had few branches. Within the organ of Corti they traveled apically (up to 70 microns) in the nerve bundles located in the IHC area before they crossed the tunnel of Corti. In the region of the OHCs, 9% of the traceable fibers branched to innervate two to three OHCs while 91% appeared to innervate only one OHC. There was no discernible difference in the distribution of contralateral and ipsilateral MOC projections in terms of cochlear region or outer hair cell rows.